1. Field of the Invention
This invention relates to a thermally insulated container. In certain aspects, the invention relates to a thermally insulated container having a thermal shield designed to conduct thermal energy to or from a heat reservoir to maintain more uniform temperature within the container.
2. Description of the Prior Art
Prior insulated containers rely on the thermal resistivity of the material comprising the container and convection currents and a heat reservoir within the container chamber to maintain a desired thermal environment within the container. A typical prior art container designed to maintain cool temperatures is a polystyrene plastic box with ice or a frozen gelpack inside the box's payload region. A significant problem with this approach is the heat flux through the box walls. Depending on the thermal resistivity of the insulation and the ambient temperature outside the box, the heat leak into the box can be significant. The resulting heat load must be convectively carried to the heat reservoir to maintain constant temperature within the box.
Note a similar problem exists in reverse if a hot product is the payload and a heat source such as a hot brick is the heat reservoir. Everything stated below will be limited to the cold payload situation, but not all embodiments of the invention are so limited.
Prior art insulated containers have proved unsuitable for products that require tight temperature tolerances. Excessive heat gain can exhaust the heat reservoir, causing the temperature to rise rapidly with additional heat gain. Temperature variation can exceed tolerances because the heat reservoir may absorb too much heat from the product itself, lowering its temperature to an unacceptable level. The temperature gradient within the payload volume may be unacceptably large because the warmer air that accumulates near the top of the container is somewhat removed from the colder air surrounding the heat reservoir. Depending on the extent of temperature gradient, a payload could conceivably be too cold at the lower end and too warm on the upper end.